The invention relates to a method for determining the free-field thrust of a gas turbine engine.
In particular the present invention concerns a method for determining the free-field thrust of a gas turbine engine carried out using an enclosed engine test facility of the kind described in our earlier UK Patent GB 2,384,058B, the disclosure of which is incorporated herein by reference.
An indoor gas turbine engine test facility is by design, a low speed wind tunnel. The wind tunnel effect is created by a secondary demand for ambient airflow as a result of the ejector pump action of the engine exhaust jet plume entering the test facility exhaust collector, otherwise called a detuner. This ejector effect creates a secondary demand for entrained/by-pass airflow that is up to five times greater than the initial airflow being demanded by the engine/intake, that is the test facility has an entrainment ratio of 5 to 1.
The creation of this wind tunnel effect is necessary to assist in expelling all undesirable hot gasses from the test cell, and to enable meaningful engine performance measurement and repeatability in a stable and consistent aerodynamic environment of non-turbulent ambient airflow. This will help eliminate any potential instability, hot gas re-ingestion or vortex formation. Also, exposed elements of test facility instrumentation/measurement systems can be cooled with ambient airflow to avoid overheating.
However, this wind tunnel effect creates a drag force acting upon the engine and its support structure, the direction of which is opposite to the thrust measured by the load measurement cells. Therefore, it is necessary to account for this thrust drag debit (typically between 1-8%) with some form of calibration, to enable measured net thrust to be corrected to a set of reference datum conditions that include still air (ISA sea level static), to obtain a corrected gross thrust.